The invention relates generally to AC power converters, typically in AC power supplies used to drive AC motors which use one or more power cells having a first converter to convert input AC power into an intermediate DC link, a capacitor connected across the intermediate DC link for smoothing, and a second converter to convert the DC power into output AC power. More particularly, the invention relates to a control method to reduce the current through the capacitor in the link between the two converters.
A power supply to control the flow of energy between a first AC system at a first frequency and first voltage and first phase count and a second AC system at a second frequency and second voltage and second phase count is commonly required in a wide variety of industrial applications, often to control the operation of an AC motor. Conventionally, such a power supply is often implemented using one or more power cells consisting of two solid state converters connected by an intermediate DC link, typically including a DC filter capacitor for smoothing. Most commonly, the phase count in both AC systems is three, but the phase counts can have any integral value, and can be different in the two AC systems.
The size of the DC filter capacitor is dependent on two factors. First, the capacitor chosen must be capable of carrying the r.m.s. current imposed on it. Second, the capacitor chosen must provide enough capacitance to hold the ripple on the DC voltage within the design limits for the application. Both of these factors can be reduced if the capacitor current is reduced. Since the DC filter capacitor is usually one of the largest and most expensive components in the power cell, it is very desirable to minimize its size and cost.
A control method and apparatus according to the invention can reduce the current through the DC filter capacitor in the link between two static converters within a power cell, thus permitting the size and cost of the capacitor to be reduced. Each static converter controls the flow of power between its AC-side and its DC-side. The capacitor current is equal to the difference between the DC-side current from the first converter and the DC-side current into the second converter. Therefore the capacitor current will be minimized if the two converter DC-side currents are caused to be as nearly equal as possible. The DC-side current into the second converter is normally determined by the requirements of the application, and therefore cannot be controlled. In such cases the capacitor current will be minimized if the DC-side current from the first converter is caused to be generally equal to the DC-side current into the second converter. Each of the two converter DC-side currents contain a steady component and one or more alternating components. The steady components will automatically be equal because no steady current can flow through the DC filter capacitor. Therefore, the alternating components should be caused to match as closely as possible. Some of the alternating components of the second converter DC-side current will be related to the switching frequency of the semiconductors in the second converter. If the first converter has a similar switching frequency, it generally may not be possible to match these components. This is because the control bandwidth of a switched-mode converter is always significantly lower than the switching frequency. However, other alternating components of the second converter DC-side current will generally be related to the operating frequency of the second AC system. If these components are sufficiently lower in frequency than the switching frequency of the first converter, then it can be possible to match these components.
The switching frequency of the first converter generally must be significantly greater than the operating frequency of the second AC system. A circuit for the first converter can be a pulse-width modulated (PWM) converter, which has a switching frequency that is independent of the operating frequency.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings illustrating certain embodiments thereof.